Two-cycle engine

ABSTRACT

Two-cycle engine includes cylinder block formed with a cylinder and crank chamber. The cylinder block includes: an exhaust passage leading to a combustion chamber in the cylinder through an exhaust port opened to an inner circumferential surface of the cylinder; a scavenging port opened to the inner circumferential surface of the cylinder; a communication passage extending from the scavenging port in a radial direction of the cylinder; and a scavenging passage extending in an axial direction of the cylinder, communicating with the crank chamber, and having an opening portion formed in a bottom surface of the communication passage. A ceiling surface of the communication passage is inclined toward a cylinder head with increasing distance from a scavenging passage side thereof toward the scavenging port. A bottom surface of the communication passage is inclined toward the crank chamber with increasing distance from a scavenging passage side thereof toward the scavenging port.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of foreign priority to JapanesePatent Application No. JP 2021-124482, filed Jul. 29, 2021, which isincorporated by reference in its entirety.

TECHNICAL FIELD

The present invention relates to a two-cycle engine, i.e., a two-strokeinternal combustion engine (hereinafter referred to as a “two-strokeengine”), used on a portable power working machine.

BACKGROUND ART

In a cylinder block of a two-stroke engine used on a portable powerworking machine such as chainsaws, brush cutters, and blowers, there areprovided with an intake passage leading to a crank chamber, an exhaustpassage leading to a combustion chamber of an upper portion of thecylinder, and a scavenging passage communicating between the crankchamber and the combustion chamber.

In a two-stroke engine, an air-fuel mixture gas (hereinafter referred toas a “mixture gas”) flows into the crank chamber through the intakepassage. Then, the mixture gas flows into the combustion chamber throughthe scavenging passage, and the mixture gas is combusted in thecombustion chamber, and by an expansion power of the mixture gas whencombusted in the combustion chamber, a piston is reciprocated in thecylinder (for example, see JP 2009-002311 A).

In the above-described two-stroke engine, when the piston descends afterthe combustion of the mixture gas, an exhaust port of the exhaustpassage opens up to the upper portion of the cylinder, and thepost-combustion gas in the combustion chamber is exhausted to theexhaust passage (exhaust process).

When the piston descends further, a scavenging port of the scavengingpassage is opened to the upper portion of the cylinder, and the mixturegas in the crank chamber flows into the combustion chamber through thescavenging passage (scavenging process).

In the scavenging process of the two-stroke engine, because both theexhaust port and the scavenging port are opened to the cylinder,unburned mixture gas flowed into the combustion chamber from thescavenging port is also exhausted to the exhaust port together with thepost-combustion gas in the combustion chamber. When the amount ofunburned gas contained in the exhaust gas increases, the amount ofhydrocarbon (HC) contained in the exhaust gas increases.

The present invention has been developed to solve the above-describedproblem, and an object of the invention is to provide a two-strokeengine that enables the unburned gas contained in the exhaust gas to bereduced, and the scavenging efficiency and the combustion efficiency tobe improved.

SUMMARY

In order to attain the above-described object, the present inventionprovides a two-stroke engine comprising: a cylinder block formed with acylinder and a crank chamber; and a piston slidably mounted in thecylinder. The cylinder block includes: an exhaust passage leading to acombustion chamber in the cylinder through an exhaust port opened to aninner circumferential surface of the cylinder; a scavenging port openedto the inner circumferential surface of the cylinder; a communicationpassage extending from the scavenging port in a radial direction of thecylinder; and a scavenging passage extending in an axial direction ofthe cylinder, communicating with the crank chamber, and having anopening portion formed in a bottom surface of the communication passage.A ceiling surface of the communication passage is inclined toward acylinder head with increasing distance from a scavenging passage sidethereof toward the scavenging port. Further, a bottom surface of thecommunication passage is inclined toward the crank chamber withincreasing distance from a scavenging passage side thereof toward thescavenging port.

According to the two-stroke engine, the communication passage betweenthe scavenging passage and the scavenging port is expanded in the axialdirection of the cylinder with increasing distance from a scavengingpassage side thereof toward the scavenging port. With thisconfiguration, the mixture gas compressed in the scavenging passage isexpanded to a large extent in the communication passage and jetted fromthe scavenging port into the combustion chamber, so that the mixture gascan be dispersed widely in the combustion chamber. This can improve thescavenging efficiency and the combustion efficiency. Further, since ascavenging flow (mixture gas) is expanded in the communication passage,atomization of the mixture gas can be promoted and the mixture gas canbe guided into the combustion chamber while spreading in the axialdirection of the cylinder. Because the scavenging flow (mixture gas)flows equally in the combustion chamber, the entire range of thecylinder can be sufficiently replaced with newly supplied mixture gas.Accordingly, the two-stroke engine according to the present inventioncan improve power output and response, while significantly reducing theamount of hydrocarbon (HC) contained in the exhaust gas.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of an engine according to oneembodiment of the present invention, illustrating an intake andcompression process viewed from an intake passage.

FIG. 2 is a cross-sectional side view illustrating the intake andcompression process of the engine according to this embodiment.

FIG. 3 is a cross-sectional view of the engine according to thisembodiment, illustrating a scavenging process viewed from the intakepassage.

FIG. 4 is a cross-sectional side view of the engine according to thisembodiment, illustrating the scavenging process.

FIG. 5 is a cross-sectional view of the engine according to thisembodiment taken on the line V-V of FIG. 4 , illustrating the scavengingprocess.

DETAILED DESCRIPTION

An embodiment of the present invention will be described in detail belowwith reference to the drawings as appropriate.

As seen in FIG. 1 , an engine 1 according to this embodiment is atwo-stroke engine used for portable power working machines such aschainsaws, brush cutters, and blowers.

The configurations of various engine mechanisms in the engine 1according to this embodiment are the same as those of known two-strokeengines, and thus the detailed descriptions for other than specificconfigurations constituting the present invention are omitted.

As seen in FIG. 2 , the engine 1 includes a cylinder block 60 formedwith a cylinder 61 a and a crank chamber 62 a, a piston 50 slidablymounted in the cylinder 61 a, and a crank shaft 90 arranged in the crankchamber 62 a.

The engine 1 further includes an intake passage 70 leading to the crankchamber 62 a, an exhaust passage 80 leading to a combustion chamber 40,and a first scavenging passage 10A and second scavenging passages 10B(see FIG. 5 ) that make the crank chamber 62 a and the combustionchamber 40 communicate with each other.

In the above-described engine 1, when the piston 50 ascends in thecylinder 61 a, inside the crank chamber 62 a is negative pressured, anda mixture gas of fuel and air produced in a carburetor (not shown) fillsup the crank chamber 62 a through the intake passage 70.

When the piston 50 reaches a top dead point, a mixture gas flowed in thecylinder 61 a in a scavenging process of a previous combustion cycle iscompressed in the combustion chamber 40. When the mixture gas is ignitedby an ignition plug 41, the piston 50 is then pushed downwards by anexpansion power of the mixture gas.

When the piston 50 descends, as seen in FIG. 4 , the exhaust passage 80is in communication with the combustion chamber 40, and thepost-combustion gas is exhausted to the exhaust passage 80. Further, asseen in FIG. 3 , by the descent of the piston 50, the mixture gas filledin the crank chamber 62 a is compressed.

When the piston 50 reaches a bottom dead point, as seen in FIG. 5 , thefirst scavenging passage 10A and the second scavenging passage 10B arein communication with the combustion chamber 40, and the mixture gasflows into the combustion chamber 40 through the first scavengingpassage 10A and the second scavenging passage 10B.

As seen in FIG. 3 , the piston 50 reaching the bottom dead point thenascends again by the torque of the crank shaft 90, and thus the intakeand compression process is repeated.

As seen in FIG. 1 , the cylinder block 60 is divided into a cylinderhead 63, an upper block 61 formed with the cylinder 61 a and an upperportion of the crank chamber 62 a, and a lower case 62 formed with alower portion of the crank chamber 62 a. The upper block 61 and thelower case 62 are assembled one above the other.

Formed on the crank shaft 90 are a crank journal 91 rotatably supportedon the lower case 62 and a crank web 92 formed on the crank journal 91.

Bearings 62 d are fitted into inner circumferential surfaces ofinserting holes 62 c formed in sidewall portions 62 b of the lower case62.

The crank journal 91 is inserted into the bearings 62 d, and the leadingends of the crank journal protrude outside from the lower case 62.

The crank web 92 is coupled with the piston 50 via a connecting rod 51,and the crank web 92 is configured to rotate around an axis of the crankjournal 91 in response to the reciprocation of the piston 50.

As seen in FIG. 2 , the intake passage 70 is formed on a side portion (aright side portion in FIG. 2 ) of the upper block 61, and one endthereof is opened to a lower portion of the cylinder 61 a and the otherend thereof is connected to a fuel supply passage (not shown).

As seen in FIG. 4 , an opening portion 71 of the intake passage 70 thatis closer to the cylinder 61 a is blocked by a side surface of thepiston 50 when the piston 50 is positioned at the bottom dead point, andis opened, as seen in FIG. 2 , to the lower portion of the cylinder 61 acommunicating with the crank chamber 62 a when the piston 50 ispositioned at the top dead point.

The exhaust passage 80 is formed on a side portion (a left side portionin FIG. 2 ) of the upper block 61 at a position opposite to the intakepassage 70. One end of the exhaust passage 80 is formed with an exhaustport 81 opened to an inner circumferential surface of the cylinder 61 a,and the other end thereof is connected to a muffler or silencer (notshown).

As seen in FIG. 4 , the exhaust port 81 is in communication with thecombustion chamber 40 when the piston 50 is positioned at the bottomdead point, and is blocked, as seen in FIG. 2 , by a side surface of thepiston 50 when the piston 50 is positioned at the top dead point.

As seen in FIG. 5 , the first scavenging passage 10A and the secondscavenging passage 10B are formed along an axial direction of thecylinder 61 a (see FIG. 1 ) at positions lateral to the cylinder 61 a(upper lateral and lower lateral in FIG. 5 ) in the upper block 61.

On a side offset from a center position P1 of the cylinder 61 a towardthe exhaust port 81, the first scavenging passage 10A and the secondscavenging passage 10B are formed in a pair across the cylinder 61 a atpositions upper and lower sides in FIG. 5 .

In this embodiment, the first scavenging passage 10A is formed on theupper side in FIG. 5 , and the second scavenging passage 10B is formedon the lower side in FIG. 5 .

Further, on a side offset from the center position P1 of the cylinder 61a toward the intake passage 70, two second scavenging passages 10B, 10Bare formed in a pair across the cylinder 61 a at positions upper andlower sides in FIG. 5 .

As seen in FIG. 1 , the first scavenging passage 10A communicates withthe crank chamber 62 a at a lower end portion of the first scavengingpassage 10A. Formed on an upper end portion of the first scavengingpassage 10A is an opening portion 11 that is opened to a bottom surface31 of a first communication passage 30A to be described later.

It should be noted that in FIGS. 1 and 3 , the first scavenging passage10A, the first communication passage 30A, and the first scavenging port20A are depicted on the cross section of the center position of thecylinder block 60 for the purpose of easily understanding configurationsof the first scavenging passage 10A, the first communication passage30A, and the first scavenging port 20A.

As seen in FIG. 1 , similar to the first scavenging passage 10A, thesecond scavenging passage 10B communicates with the crank chamber 62 aat a lower end portion of the second scavenging passage 10B (see FIG. 1), and an upper end portion of the second scavenging passage 10B has theopening portion 11 formed to be opened to the bottom surface 31 of asecond communication passage 30B to be described later.

It should be noted that in FIGS. 1 and 3 , the second scavenging passage10B, the second communication passage 30B, and the second scavengingport 20B are depicted on the cross section of the center position of thecylinder block 60 for the purpose of easily understanding configurationsof the second scavenging passage 10B, the second communication passage30B, and the second scavenging port 20B.

As seen in FIG. 1 , the first scavenging passage 10A is formed such thatan inner surface thereof located at an outer side in the radialdirection of the cylinder 61 a is straight along the axial direction ofthe cylinder 61 a. Further, the first scavenging passage 10A is formedsuch that an inner surface located closer to the cylinder 61 a at aposition 12 closer to the first communication passage 30A (the openingportion 11) is arranged more radially outside of the cylinder 61 a thanan inner surface located closer to the cylinder 61 a at a position 13closer to the crank chamber 62 a. In other words, the first scavengingpassage 10A is formed such that the inner surface located closer to thecylinder 61 a at a position 12 closer to the first communication passage30A has a protrusion formed to protrude farther outward in the radialdirection of the cylinder 61 a than the inner surface located closer tothe cylinder 61 a at a position closer to the crank chamber 62 a.

This allows the cross-sectional area of the first scavenging passage 10Ain the radial direction of the cylinder 61 a to be formed smaller at aposition 12 closer to the first communication passage 30A (the openingportion 11) than at a position 13 closer to the crank chamber 62 a.

The first scavenging port 20A and the second scavenging port 20B areopening portions each having a rectangular cross-section opened to theinner circumferential surface of the cylinder 61 a (see FIG. 4 ).

The first scavenging port 20A and the second scavenging port 20B thatare positioned opposite to each other across the cylinder 61 a areformed at positions on both sides of the exhaust port 81 (upper side andlower side in FIG. 5 ) on a side offset from the center position P1 ofthe cylinder 61 a toward the exhaust port 81.

In this embodiment, the first scavenging port 20A is formed on the upperside in FIG. 5 , and the second scavenging port 20B is formed on thelower side in FIG. 5 .

Further, two second scavenging ports 20B, 20B that are positionedopposite to each other across the cylinder 61 a are formed at positionson both sides of the exhaust port 81 on a side offset from the centerposition P1 of the cylinder 61 a toward the intake passage 70.

As seen in FIG. 4 , the first scavenging port 20A and the secondscavenging port 20B are opened to the inner circumferential surface ofthe cylinder 61 a at about the same height as that of the exhaust port81.

Therefore, the first scavenging port 20A and the second scavenging port20B are opened to the upper portion of the cylinder 61 a and incommunication with the combustion chamber 40 when the piston 50 ispositioned at the bottom dead point.

Further, as seen in FIG. 2 , the first scavenging port 20A and thesecond scavenging port 20B are blocked by the side surface of the piston50 when the piston 50 is positioned at the top dead point.

As seen in FIG. 5 , the first communication passage 30A and the secondcommunication passage 30B are passageways formed in a radial directionof the cylinder 61 a.

On a side offset from the center position P1 of the cylinder 61 a towardthe exhaust port 81, the first communication passage 30A and the secondcommunication passage 30B are symmetrically formed at positions upperand lower sides in FIG. 5 across the cylinder 61 a. In this embodiment,the first communication passage 30A is formed on the upper side in FIG.5 , and the second communication passage 30B is formed on the lower sidein FIG. 5 .

Further, on a side offset from the center position P1 of the cylinder 61a toward the intake passage 70, two second communication passages 30B,30B are formed in a pair across the cylinder 61 a at positions upper andlower sides in FIG. 5 .

An opening portion 11 of the first scavenging passage 10A is opened inthe bottom surface 31 of the first communication passage 30A. The firstcommunication passage 30A is a passageway communicating between theopening portion 11 and the first scavenging port 20A. A landing portionin the shape of landing is formed between the opening portion 11 and thefirst scavenging port 20A by the bottom surface 31 of the firstcommunication passage 30A. The landing portion serves as a guidingsurface for guiding the mixture gas from below (see FIG. 1 ).

An opening portion 11 of the second scavenging passage 10B is opened inthe bottom surface 31 of the second communication passage 30B. Thesecond communication passage 30B is a passageway communicating betweenthe opening portion 11 and the second scavenging port 20B. A landingportion in the shape of a landing is formed between the opening portion11 and the second scavenging port 20B by the bottom surface 31 of thesecond communication passage 30B. The landing portion serves as aguiding surface for guiding the mixture gas from below (see FIG. 1 ).

The first communication passage 30A and the second communication passage30B are formed from the opening portions 11 formed in the bottomsurfaces 31 toward a far side from the exhaust port (hereinafterreferred to as a counter-exhaust port side) that is a side opposite tothe exhaust port 81 (i.e., a side closer to the intake passage 70) inthe combustion chamber 40 (cylinder 61 a).

Consequently, the mixture gas flowed into the first communicationpassage 30A from the opening portion 11 of the first scavenging passage10A is guided toward the counter-exhaust port side by the firstcommunication passage 30A, and jetted (injected) in the combustionchamber 40 in a direction toward the counter-exhaust port side from thefirst scavenging port 20A.

Similarly, the mixture gas flowed into the second communication passage30B from the opening portion 11 of the second scavenging passage 10B isguided toward the counter-exhaust port side by the second communicationpassage 30B, and jetted (injected) in the combustion chamber 40 in adirection toward the counter-exhaust port side from the secondscavenging port 20B.

As seen in FIG. 4 , the first communication passage 30A has arectangular cross-section that is formed with the bottom surface 31,both side surfaces 32, 33, and a ceiling surface 34.

As seen in FIG. 5 , among the side surfaces 32, 33 that form the firstcommunication passage 30A, the side surface 33 located at thecounter-exhaust port side is inclined from the outside in the radialdirection of the cylinder 61 a toward the first scavenging port 20A inthe upper block 61 so as to be away from the other side surface 32located closer to the exhaust port 81. In other words, the side surface33 of the first communication passage 30A that is located at thecounter-exhaust port side is formed directed toward the far side fromthe exhaust port (counter-exhaust port side) in the combustion chamber40 (cylinder 61 a).

The opening width of the first communication passage 30A in acircumferential direction of the cylinder 61 a is expanded from theopening portion 11 toward the first scavenging port 20A.

As seen in FIG. 1 , the ceiling surface 34 of the first communicationpassage 30A is inclined toward the cylinder head 63 (upper side in FIG.1 ) with increasing distance from the opening portion 11 toward thefirst scavenging port 20A.

The bottom surface 31 of the first communication passage 30A is inclinedtoward the crank chamber 62 a (lower side in FIG. 1 ) with increasingdistance from the opening portion 11 toward the first scavenging port20A.

As described above, the height of the first communication passage 30A(i.e., width of the first communication passage 30A in the axialdirection of the cylinder 61 a) is gradually increased toward the firstscavenging port 20A from a position outside in the radial direction ofthe cylinder 61 a.

It is preferable that an inclination angle of the bottom surface 31 ofthe first communication passage 30A toward the crank chamber 62 a is setin the range of 70 to 100% of an inclination angle of the ceilingsurface 34 of the first communication passage 30A toward the cylinderhead 63.

It is preferable that the maximum height (maximum width) of the firstscavenging port 20A in the axial direction of the cylinder 61 a is equalto or more than twice the minimum height (minimum width) of the firstcommunication passage 30A in the axial direction of the cylinder 61 a.

Further, it is preferable that a ratio of the minimum height of thefirst communication passage 30A in the axial direction of the cylinder61 a to the maximum height of the first scavenging port 20A in the axialdirection of the cylinder 61 a is set in the range of 1:2 to 1:4.

It is preferable that the opening area of the first scavenging port 20Ais equal to or more than four times the minimum cross-sectional area ofthe first communication passage 30A in the axial direction of thecylinder 61 a.

It is preferable that a ratio of the minimum cross-sectional area of thefirst communication passage 30A in the axial direction of the cylinder61 a to the opening area of the first scavenging port 20A is set in therange of 1:4 to 1:8.

The bottom surface 31 of the first communication passage 30A and aninner wall surface of the first scavenging passage 10A in the radialdirection of the cylinder 61 a (wall surface of the first scavengingpassage 10A at a position closer to the first communication passage) arecontinuous through an arc-shaped curved surface 35. Further, the ceilingsurface 34 of the first communication passage 30A and the inner surfaceof the first scavenging passage 10A are continuous through an arc-shapedcurved surface 36.

The first communication passage 30A according to this embodiment isformed in a divergent form in which the cross-sectional area of thepassage gradually expands from the opening portion 11 toward the firstscavenging port 20A.

The cross-sectional area of the first communication passage 30A in theaxial direction of the cylinder 61 a is made larger than thecross-sectional area of the first scavenging passage 10A in the radialdirection of the cylinder 61 a at a position 12 closer to the firstcommunication passage 30A (opening portion 11).

Similar to the first communication passage 30A, as seen in FIG. 4 , thesecond communication passage 30B has a rectangular cross-section that isformed with the bottom surface 31, both side surfaces 32, 33, and aceiling surface 34.

As seen in FIG. 5 , the side surface 33 of the second communicationpassage 30B that is located at the counter-exhaust port side is alsoformed directed toward the far side from the exhaust port(counter-exhaust port side) in the combustion chamber 40.

Further, as seen in FIG. 1 , the ceiling surface 34 of the secondcommunication passage 30B is also inclined toward the cylinder head 63(upper side in FIG. 1 ) with increasing distance from the openingportion 11 toward the second scavenging port 20B.

The second communication passage 30B is formed such that thecross-sectional area thereof is made larger with increasing distancefrom the opening portion 11 toward the second scavenging port 20B. Thecross-sectional area of the second communication passage 30B in theaxial direction of the cylinder 61 a is made larger than thecross-sectional area of the second scavenging passage 10B in the radialdirection of the cylinder 61 a at the opening portion 11.

As seen in FIG. 3 , according to the engine 1 of this embodiment, whenthe piston 50 reaches the bottom dead point, the first scavenging port20A and the second scavenging port 20B are in communication with thecombustion chamber 40.

Accordingly, the mixture gas having been filled in the crank chamber 62a flows into the combustion chamber 40 through the first scavengingpassage 10A, the first communication passage 30A, and the firstscavenging port 20A.

Further, the mixture gas having been filled in the crank chamber 62 aflows into the combustion chamber 40 through the second scavengingpassages 10B, the second communication passages 30B, and the secondscavenging ports 20B.

According to the engine 1 of this embodiment, the ceiling surface 34 ofthe first communication passage 30A is inclined toward the cylinder head63 with increasing distance from the first scavenging passage sidethereof to the first scavenging port 20A. Further, the bottom surface 31of the first communication passage 30A is inclined toward the crankchamber 62 a with increasing distance from the first scavenging passageside thereof to the first scavenging port 20A.

With this configuration, the mixture gas compressed in the firstscavenging passage 10A is expanded to a large extent in the firstcommunication passage 30A and jetted (injected) from the firstscavenging port 20A into the combustion chamber 40. The mixture gas isdispersed in the combustion chamber 40 with respect to the axialdirection of the cylinder 61 a. As compared with the alternativeconfiguration in which the width of the first communication passage 30Ain the axial direction of the cylinder 61 a is made constant,atomization of the mixture gas can be promoted upon expansion of themixture gas and homogeneity of the component of the mixture gas can beimproved within the combustion chamber 40. This can stabilize combustionof the mixture gas.

Since the inclination angle of the bottom surface 31 of the firstcommunication passage 30A toward the crank chamber 62 a is set in therange of 70 to 100% of the inclination angle of the ceiling surface 34of the first communication passage 30A toward the cylinder head 63 toslightly extend the first communication passage 30A toward thecombustion chamber 40, the mixture gas can efficiently flow into thecombustion chamber 40. It is particularly preferable that theinclination angle of the bottom surface 31 of the first communicationpassage 30A toward the crank chamber 62 a is set to be 90% of theinclination angle of the ceiling surface 34 of the first communicationpassage 30A toward the cylinder head 63.

As seen in FIG. 5 , the opening width of the first communication passage30A in the circumferential direction of the cylinder 61 a is graduallyexpanded toward the first scavenging port 20A from a position closer tothe opening portion 11 of the first scavenging passage 10A.

According to the engine 1 of this embodiment, as seen in FIG. 3 , thefirst communication passage 30A is gradually expanded both in the axialdirection and the circumferential direction of the cylinder 61 a, from afirst scavenging passage side toward the first scavenging port 20A. Inother words, the first communication passage 30A is formed in adivergent form in which the cross-sectional area of the passagegradually expands from the opening portion 11 toward the firstscavenging port 20A.

With this configuration, since the cross-sectional area of the firstcommunication passage 30A expands in the circumferential direction ofthe cylinder 61 a as well as in the axial direction of the cylinder 61a, the mixture gas compressed in the first scavenging passage 10A isexpanded to a large extent in the first communication passage 30A andjetted (injected) from the first scavenging port 20A into the combustionchamber 40 so that the mixture gas can be dispersed widely in thecombustion chamber 40. This can improve the scavenging efficiency andthe combustion efficiency.

As seen in FIG. 1 , it is preferable that the maximum height (maximumwidth) of the first scavenging port 20A in the axial direction of thecylinder 61 a is equal to or more than twice the minimum height (minimumwidth) of the first communication passage 30A in the axial direction ofthe cylinder 61 a.

Further, it is more preferable that the ratio of the minimum height ofthe first communication passage 30A in the axial direction of thecylinder 61 a to the maximum height of the first scavenging port 20A inthe axial direction of the cylinder 61 a is set in the range of 1:2 to1:4.

Setting the maximum height of the first scavenging port 20A and theminimum height of the first communication passage 30A in this rangemakes it possible to efficiently improve the scavenging efficiency andthe combustion efficiency.

It is preferable that the opening area of the first scavenging port 20Ais equal to or more than four times the minimum cross-sectional area ofthe first communication passage 30A in the axial direction of thecylinder 61 a.

Further, it is preferable that the ratio of the minimum cross-sectionalarea of the first communication passage 30A in the axial direction ofthe cylinder 61 a to the opening area of the first scavenging port 20Ais set in the range of 1:4 to 1:8.

Setting the minimum cross-sectional area of the first communicationpassage 30A and the opening area of the first scavenging port 20A inthis range makes it possible to efficiently improve the scavengingefficiency and the combustion efficiency.

According to the engine 1 of this embodiment, the first scavengingpassage 10A is formed such that the inner surface located closer to thecylinder 61 a at a position 12 closer to the first communication passage30A is arranged more radially outside of the cylinder 61 a than theinner surface located closer to the cylinder 61 a at a position closerto the crank chamber 62 a. With this configuration, the bottom surface31 and the ceiling surface 34 of the first communication passage 30Athat serve as guiding surfaces for guiding the mixture gas are set tohave sufficient lengths thereof, so that the directivity and theatomization-promoting efficiency of the mixture gas can be enhanced.Further, the cross-sectional area of the first scavenging passage 10A inthe radial direction of the cylinder 61 a is smaller at a position 12closer to the first communication passage 30A than at a position 13closer to the crank chamber 62 a.

With this configuration, the mixture gas having flowed from the crankchamber 62 a into the first scavenging passage 10A is compressed in thefirst scavenging passage 10A, and then flows into the cylinder 61 awhile expanding in the first communication passage 30A. As describedabove, since the mixture gas is once compressed and then expanded topromote the atomization of the mixture gas and the mixture gas hasdirectivity in the axial direction of the cylinder 61 a, it is possibleto scavenge the inner side of the cylinder 61 a equally. Accordingly,since the mixture gas is introduced equally throughout the entire rangeof the inner side of the cylinder 61 a, the post-combustion gas at theprevious combustion cycle is swept out and discharged, and thepost-combustion gas is replaced with the mixture gas required for thecurrent combustion cycle without any waste. Therefore, the output andthe response of the engine 1 can be significantly improved and theemission of unburned gas can be reduced.

Further, the cross-sectional area of the first scavenging passage 10A inthe radial direction of the cylinder 61 a at a position 12 closer to thefirst communication passage 30A is smaller than the cross-sectional areaof the first communication passage 30A in the axial direction of thecylinder 61 a.

With this configuration, the mixture gas flowed from the firstscavenging passage 10A into the first communication passage 30A throughthe opening portion 11 is once compressed in the first scavengingpassage 10A, and then expanded in the first communication passage 30A.

Further, according to the engine 1 of this embodiment, the inner surfaceof the first scavenging passage 10A and the bottom surface 31 of thefirst communication passage 30A are continuous through the arc-shapedcurved surface 35. Further, the ceiling surface 34 of the firstcommunication passage 30A and the inner surface of the first scavengingpassage 10A are continuous through the arc-shaped curved surface 36.

With this configuration, when the mixture gas flows from the firstscavenging passage 10A into the first communication passage 30A, themixture gas can flow smoothly along the curved surfaces 35, 36.

According to the engine 1 of this embodiment, as seen in FIG. 5 , theside surfaces 33 of the first communication passage 30A and the secondcommunication passage 30B that are located at the counter-exhaust portside are formed directed toward the counter-exhaust port side in thecombustion chamber 40. Further, the cylinder block 60 of the engine 1according to this embodiment has a plurality of the scavenging ports20A, 20B, and the first communication passage 30A is formed, among theplurality of the scavenging ports 20A, 20B, from the first scavengingport 20A disposed closer to the exhaust port 81.

With this configuration, since the mixture gases flowed from the firstscavenging port 20A and the second scavenging port 20B into thecombustion chamber 40 are directed to the counter-exhaust port side, theamount of unburned mixture gas exhausted into the exhaust port 81 can besignificantly reduced.

As described above, the engine 1 according to this embodiment cansignificantly reduce the amount of hydrocarbon (HC) contained in theexhaust gas.

As compared with a conventional engine having no first communicationpassage 30A, it was shown that the engine 1 according to this embodimentcan reduce the amount of hydrocarbon contained in the exhaust gas byabout 30%.

Although one preferred embodiment of the present invention has beendescribed in detail, it is understood that the present invention is notlimited to the above specific embodiment and various changes andmodifications may be made where appropriate without departing from thegist and scope of the present invention.

As seen in FIG. 5 , the engine 1 according to this embodiment isconfigured such that one communication passage among four communicationpassages is formed in the shape of the first communication passage 30A.However, the number of first communication passages 30A is not limited.For example, all of the four communication passages may be formed as thefirst communication passage 30A.

It is preferable that the configuration of the present invention isapplied to one scavenging port among adjacent or opposite scavengingports. With this configuration, since the mixture gases from scavengingports that are arranged closely to each other flow into the cylinder 61a with different directivities and at different speeds, the directivityof each mixture gas can be maintained while avoiding collision of themixture gases. This can efficiently improve the scavenging efficiencyand the combustion efficiency of the engine 1.

According to the engine 1 of this embodiment, four scavenging ports areformed. However, the number of scavenging ports is not limited. Forexample, two scavenging ports may be formed one on each side of theexhaust port 81.

1. A two-stroke engine comprising: a cylinder block formed with acylinder and a crank chamber; and a piston slidably mounted in thecylinder, wherein the cylinder block includes: an exhaust passageleading to a combustion chamber in the cylinder through an exhaust portopened to an inner circumferential surface of the cylinder; a scavengingport opened to the inner circumferential surface of the cylinder; acommunication passage extending from the scavenging port in a radialdirection of the cylinder; and a scavenging passage extending in anaxial direction of the cylinder, communicating with the crank chamber,and having an opening portion formed in a bottom surface of thecommunication passage, wherein a ceiling surface of the communicationpassage is inclined toward a cylinder head with increasing distance froma scavenging passage side thereof toward the scavenging port, and abottom surface of the communication passage is inclined toward the crankchamber with increasing distance from a scavenging passage side thereoftoward the scavenging port.
 2. The two-stroke engine according to claim1, wherein a maximum width of the scavenging port in the axial directionof the cylinder is equal to or more than twice a minimum width of thecommunication passage in the axial direction of the cylinder.
 3. Thetwo-stroke engine according to claim 1, wherein a ratio of a minimumwidth of the communication passage in the axial direction of thecylinder to a maximum width of the scavenging port in the axialdirection of the cylinder is set in the range of 1:2 to 1:4.
 4. Thetwo-stroke engine according to claim 1, wherein an opening area of thescavenging port is equal to or more than four times a minimumcross-sectional area of the communication passage in the axial directionof the cylinder.
 5. The two-stroke engine according to claim 1, whereina ratio of a minimum cross-sectional area of the communication passagein the axial direction of the cylinder to an opening area of thescavenging port is set in the range of 1:4 to 1:8.
 6. The two-strokeengine according to claim 1, wherein an inclination angle of the bottomsurface of the communication passage toward the crank chamber is set inthe range of 70 to 100% of an inclination angle of the ceiling surfaceof the communication passage toward the cylinder head.
 7. The two-strokeengine according to claim 1, wherein a cross-sectional area of thescavenging passage in the radial direction of the cylinder is smaller ata position closer to the communication passage than at a position closerto the crank chamber, and the cross-sectional area of the scavengingpassage in the radial direction of the cylinder at the position closerto the communication passage is smaller than a cross-sectional area ofthe communication passage in the axial direction of the cylinder.
 8. Thetwo-stroke engine according to claim 7, wherein the scavenging passageis formed such that an inner surface located closer to the cylinder at aposition closer to the communication passage is arranged more radiallyoutside of the cylinder than an inner surface located closer to thecylinder at a position closer to the crank chamber.
 9. The two-strokeengine according to claim 1, wherein an inner surface of the scavengingpassage and the bottom surface of the communication passage arecontinuous through an arc-shaped curved surface.
 10. The two-strokeengine according to claim 1, wherein a side surface located at acounter-exhaust port side that forms the communication passage is formeddirected toward the counter-exhaust port side in the combustion chamber.11. The two-stroke engine according to claim 1, wherein an opening widthof the communication passage in a circumferential direction of thecylinder is gradually expanded toward the scavenging port from aposition closer to the opening portion of the scavenging passage. 12.The two-stroke engine according to claim 1, wherein the cylinder blockhas a plurality of the scavenging ports and at least one thecommunication passage, and the communication passage is formed, amongthe plurality of the scavenging ports, from one scavenging port disposedcloser to the exhaust port.